<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta http-equiv="X-UA-Compatible" content="IE=edge">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>MarkDown Editor</title>
  <style>
    html,body{
      width: 100%;
      height: 100%;
      margin: 0;
      padding: 0;
    }
    #editor{
      width: 100%;
      height: 100%;
      box-sizing: border-box;
      padding: 20px;
      border:1px solid #333;
    }
  </style>
</head>
<body>
  <div id="editor" contenteditable="true"></div>
</body>
</html>